>In English, a double negative is a positive (I don't have no answer = I have
>an answer).

I really don't think that "I ain't got no answer" really does not mean "I
have an answer" in English, at least not here in North Carolina, where my
parents live in Kentucky, or where I student taught in inner city Flint.

Double negatives are considered poor style in formal writing. My English
teacher in elementary school told us that a double negative is a positive
in order to convince us that double negatives are illogical and confusing.
This is silly. In formal writing, double negatives are discouraged. In many
forms of colloquial English, double negatives are common, and they do not
negate each other.

>Luke 22:67b EAN hUMIN EIPW OU MH PISTEUSHTE:
>I guess in Greek it is a way of emphasizing (that is what it
>looks like, but I am probably wrong again).

No, I think it is best to think of OU MH as a unit, as one emphatic negative.

>Ergo, could it
>be (rather literally) translated: "No, you would not believe [even] if I
>told you"? That is, to paraphrase: I reject the very idea of saying it
>plainly, because in fact you will not believe it whatever what I say.

Zerwick's grammar, section 444, has an interesting discussion of OU MH. He
says that this was used in classical Greek with the aorist subjunctive or
with the future indicative as an emphatic negative for the future. ("If I
told you, you would never believe it".)

He says that in the Septuagint, the Hebrew LO is translated by OU or OU MH
indiscriminately. In the papyri, OU MH is rare, and always very emphatic.

Outside of Revelation, where it occurs 16 times, he says that this
construction is almost limited to quotations from the LXX and the words of
Jesus (57 out of the 61 occurrences in the Gospels). The phrase OU MH is
never used by the Gospel writers, or by Luke in Acts, for their own
narrative, but only in quotes. Moulton says this suggests that OU MH is
used primarily for sacred utterances. Zerwick concludes that OU MH, in the
majority of NT uses, expresses a *prophetic* emphasis.

I don't know how to get that across in translation. Several translations
say, "If I tell you, you will not believe", which feels a little grander
and more prophetic to me than "If I tell you, you will not believe me", but
that is a matter of feel and taste.